Along with an explosive increase in demand of a broadband multimedia communication service such as the Internet or a high-definition digital TV broadcast, a dense wavelength-division multiplexing optical fiber communication system, which is suitable for a long-distance and large-capacity transmission and is highly reliable, has been introduced in trunk networks and metro networks. Also in access networks, optical fiber access services have spread rapidly. In such an optical fiber communication system, reduction in costs for laying optical fibers as optical transmission lines and improvement in spectral efficiency per optical fiber are required. Therefore, a wavelength-division multiplexing technology which multiplexes multiple optical signals having different wavelengths is widely used.
In an optical transmitter for such a high-capacity wavelength-division multiplexing communication system, an optical modulator is a key component. In the optical modulator, high speed operation with small wavelength dependence is indispensable. Further, an unwanted optical phase modulation component (in the case of generating an optical intensity modulation signal) or an optical intensity modulation component (in the case of generating an optical phase modulation signal) which degrades the waveform of the received optical signal after long-distance transmission should be suppressed as small as possible. A Mach-Zehnder (MZ) optical intensity modulator in which a couple of waveguide type optical phase modulators are embedded into an MZ interferometer is suitable for such a use.
A waveguide type semiconductor optical phase modulator or a semiconductor MZ optical modulator using a III-V compound semiconductor such as gallium arsenide (GaAs) or indium phosphide (InP) in a light source are expected to realize downsizing and cost reduction by monolithic integration. Downsizing and cost reduction are significantly important to put an optical transmitter for a wavelength-division multiplexing optical fiber communication system into practical use.
By the way, a wavelength-tunable light source generally includes a resonator structure in which a reflector having a wavelength-tunable mechanism and a reflector having small (or no) wavelength dependence are arranged at both ends of a gain region which generates signal light. The reflector having the above-mentioned wavelength-tunable mechanism can be categorized into two types. One is a reflector in which a semiconductor optical waveguide type ring optical resonator or a Bragg reflector is monolithically integrated with a gain region. The other is a reflector in which a diffraction grating having wavelength selectivity is provided outside and coupled to a gain region by a lens. As the reflector having small wavelength dependence, a cleavage surface of the semiconductor optical waveguide can be used.
When the waveguide type semiconductor optical modulator is monolithically integrated with the wavelength-tunable light source, it is necessary to provide a reflector having small wavelength dependence between them. The reflector is required to reflect a certain amount of light and couple the signal light from the wavelength-tunable light source to the optical modulator at a practical rate. Further, in order that the optical modulator region and the wavelength-tunable light source region, which are monolithically integrated, operate independently, or in order that each control signal does not influence other control signals, the optical modulator region and the wavelength-tunable light source region should be electrically isolated from each other. Thus, there is proposed a structure (narrow gap mirror) in which a gap with a length of about a signal light wavelength is formed in the optical waveguide connecting the wavelength-tunable light source and the optical modulator which are monolithically integrated (refer to Non-Patent Document 1, for example). The narrow gap mirror can be formed by dry etching technology.
However, there are some problems in putting the narrow gap mirror into practical use. Firstly, the wavelength-tunable light source and the optical modulator are required to be directly aligned in a longitudinal direction. Therefore, the monolithically integrated device tends to be long, which is a disadvantage in downsizing, fabrication yield, and the like. Further, a high processing accuracy of about 0.1 μm is required.
Further, it is difficult to control an etching depth because an etch stop mask is required to cover the surface of a semiconductor substrate at an extremely high rate. In addition, acceptance/rejection determination is difficult because it is difficult to observe the inside of an etched gap.
Further, in a discontinuous portion of the optical waveguide such as the gap mirror, the signal light is scattered. Therefore, it is difficult to improve the signal light coupling efficiency between the wavelength-tunable light source region and the optical modulator region. The signal light scattered by such a gap mirror is transmitted to the vicinity of a signal light output end face through the semiconductor substrate as non-guided light. Thus, extinction properties as an optical modulator are degraded. For example, to prevent such scattering, a resin having a proper refractive index may be filled into the narrow gap mirror. However, heat treatment of the resin and optimization of forming conditions for an underlaying film are difficult. Thus, there are some problems in correction of reflection-transmission characteristics and reliability. Further, there is a concern about contamination of a process unit by the resin. In addition, the reflection-transmission characteristics are determined in a preceding process. Thus, adjustment and yield recovery are difficult in a later process.
Further, exposing the worked surface (the end face of the optical waveguide) at which the narrow gap mirror is formed leads to the reliability problem. On the other hand, it is difficult to form a dielectric protection film on the worked surface in view of controlling a coating shape and uniformity.
[Non-Patent Document 1] S. Sudo and six others, “External Cavity Wavelength Tunable Laser Utilizing On-Chip VOA”, Technical Digest of Optical Fiber Communication Conference and Exposition 2006, OWL4